Substance use among adolescents continues to grow and is a major healthcare problem. Very little research has been done to study the neurobiology of substance abuse/dependence in the adolescent population. Understanding the neurobiology of substance abuse/dependence would help in developing effective preventive and/or treatment strategies. We propose to study the dopaminergic sensitization associated with the addictive process. The proposal uses a presynaptic (e.g. methylphenidate) followed by a postsynpatic (e.g. pergolide) dopamine agonist challenge test to address this question in adolescents with history of substance abuse/dependence as compared to normal matched controls.